1. Field of the Invention
The present invention is a system, method, and apparatus to improve the quality of stormwater runoff or sanitary wastewater effluent by removing pollutants, and, more particularly to a system whereby contaminated water is passed through a one or two-stage complex high flow rate mixed media treatment system. The first stage treatment uses mulch, soil particles, microbes and live plants to treat contaminated water where it flows by gravity to the second stage where pollutants are further removed by sedimentation and anaerobic microbiological processes. Under certain conditions, only the first stage treatment may be effectively utilized.
2. Background Information
Land development results in an increased stormwater runoff. The increased runoff can be as much as 5 to 10 times higher compared to pre-development conditions. This increased runoff can carry with it a variety of pollutants generated from diffuse sources. The pollutants can include sediment from construction sites and stream erosion, heavy metals, oil and grease, toxic organic and inorganic chemicals, nutrients and organic materials depending on the land use. One objective of current stormwater management programs is to remove these pollutants from the runoff prior to it being discharged to surface waters or percolating into the ground water. There are a variety of so-called xe2x80x9cbest management practicesxe2x80x9d (BMP""s) which are used to remove pollutants. Some of these include retention and detention ponds, wetlands, forested buffers, sedimentation basins, infiltration trenches, grass swales, and various types of filters using peat, sand, soil and leaf mulch and aggregates.
The enactment of the 1972 Clean Water Act (CWA) (and subsequent amendments) recognized the adverse environmental impacts of point and nonpoint pollution on the physical, chemical and biological integrity of our receiving waters. Since enactment of the CWA and the subsequent implementation of National Pollutant Discharge Elimination Systems (NPDES) permit program industries, states and local governments have been in the process of developing strategies and technologies to reduce both point and nonpoint pollution problems. Nonpoint source pollution is the term used to describe the diffuse and non-discrete sources and character of the pollution that can contaminate stormwater runoff. As stormwater runoff flows across the surface of developed land, it can become contaminated with and transport such pollutants as sediment, nitrogen, phosphorus, bacteria, heavy metals, insecticides, pesticides, herbicides, trash, debris, organic material and petroleum products. There is no one source of this nonpoint pollution. Instead this pollution comes from many sources associated with changes in land use, human activities and air pollution deposition. Point source pollution emanates from discrete easily identifiable discharges such as a pipe discharging effluent from wastewater treatment plant, factory or septic system.
High levels of point or nonpoint pollutants in surface waters will result in the degradation of the water quality to receiving surface waters (streams, rivers, lakes and reservoirs), contamination of ground water supplies as pollutant laden waters percolate in the ground and destruction of the aquatic biota (plants, fisheries and invertebrates) sensitive to poor water quality. Contaminated runoff can have a deleterious effect on the human health by degrading the quality of drinking water supplies.
Since the early 1980""s regulations have been in place requiring new development to reduce problems associated with nonpoint pollution and stormwater runoff. Numerous BMP""s have been developed to treat stormwater to capture, remove or transform pollutants thus reducing their levels in the discharge to surface waters.
The character and levels of the major constituents polluting stormwater runoff are well known and have been studied for many years. The first program to characterize pollutants and their levels in runoff was performed by the United States Environmental Protection Agency (EPA) in a multi year Nationwide Urban Runoff Program (NURP) which began in 1979. They studied 28 separate urban areas across the nation. For example, in the Washington, D.C. metropolitan area the EPA studied urban runoff over a four-year period from 1979 to 1983, and the results were published in 1983 (PO-003208-01). Table 1 summarizes some of the pollutant level contamination findings of the Washington, D.C. area study. The concentrations shown are the averages of all samples examined during the study period.
The study concluded that the pollutant levels in urban runoff represent a significant threat to the integrity of receiving waters. The high sediment loads cause excess turbidity blocking light to submerged aquatic plants. High phosphorus and nitrogen levels (nutrients for plant growth) cause excessive algae growth, which depletes the water of oxygen suffocating fish and other organisms. Bacteria levels were above public health criteria for recreational activities.
The benefit of filtering contaminated water (stormwater or wastewater) through or bringing it into contact with such constituents as soils, sand, silts, clays, organic material, microbes and plants to treat and remove pollutants from stormwater runoff and wastewater is well known. A variety of complex multimedia filters have been used in the past to remove contaminates from drinking water, wastewater and stormwater runoff. These would include grass swales, stormwater management ponds, wetlands, land spray irrigation treatment systems for wastewater, naturally vegetative buffers, sand filters and bioretention systems.
Constructed ponds incorporating shallow wetlands systems are an example of a BMP that uses a variety of physical, chemical and biological processes to treat stormwater runoff. Runoff flows through the stormwater pond where sedimentation occurs removing particles from the water column and associated pollutant such as organic materials and heavy metals attached to the particles. Runoff is exposed to the pond soils where pollutants are capture by adsorption onto organic and inorganic constituents of the soil. Biological processes occur in the water column by bacteria, algae and plants that assimilate, transform and uptake pollutants and nutrients as part of their metabolic processes. Ponds require long detention times to remove suspended particulate matter due to the time it takes for very small particles to settle out of the water column. Varying intensities of rainstorm events can cause high flow rates through the ponds reducing the time for treatment and poor designs that shorten retention times cause the pollutant removal rate of ponds to be highly variable. High storm flows can cause re-suspension of particles thus flushing out captured pollutants. Under high flow and poor design conditions, ponds have been shown to export some pollutants associated with the re-suspended soil particles.
For BMP""s such as ponds, swales and forested buffers, plants play an important role in the removal of various pollutants as they can assimilate into their tissues and incorporate into their bio-mass many of the pollutants or by-products of the break down of the pollutants accomplished by microbial decay. The pollutants would include nitrogen, phosphorous, complex hydrocarbons (oils and grease), carbon dioxide and heavy metals. The soil particles and organic material in these BMP""s act to trap and capture pollutants and nutrients and as a media for microbiological reactions to degrade or transform organic and chemical components into substances that plants can then absorb into their tissues.
A 1990 study conducted by the EPA xe2x80x9cPerformance Evaluation at a long-term Food Processing Land Treatment Site PB90-195389xe2x80x9d at a Paris, Tex. wastewater treatment plant showed that polluted effluent from a non hazardous treatment plant could be effectively treated by allowing the water to flow across and into the soils of a meadow with the pollutants being removed by the soil and plant material. This type of land treatment can operate effectively for many years. In this study, the treatment plant was in continues operation over a 24-year period. In this system, the soils consisted of clays, sand, and clay loans with organic carbon levels ranging from 0.27% to 1.72%. The plants used were reed canary grass and tall fescue. The pH values of the soil ranged from 4.65 to 7.16. Table 2 shows the concentration of some of the constituents in raw wastewater that was discharged to the land treatment filtering system.
Comparing the constituent levels in the wastewater from the 1990 EPA study to the urban runoff levels in the 1983 EPA NURP study, it is apparent that the wastewater pollutant loads were many times higher than the urban runoff levels. The 1983 EPA study on the land treatment systems showed that the pollutant removal rates for BOD (biological oxygen demand), COD (chemical oxygen demand), TOC (total organic carbon) and TSS (total suspend solids) were consistently high with mean removal rates of 92%. Total nitrogen removal rates were between 84% and 89%. This study demonstrated the effectiveness of plants/microbes/soil in removing pollutants from a source of contaminated water.
The 1990 EPA study also showed that even with the treatment areas being exposed to rain fall during the year, the system was capable of handling both the wastewater flows and the rain water runoff without affecting the performance of the systems. This highly stable system functioned effectively over a 24-year study period and maintained high pollutant removal rates of ammonia between 60 to 99 percent, TSS 87 to 95 percent and BOD 90 to 99 percent. The rate of application of sewage effluent on the meadow was quite low and using this type of system for stormwater runoff would require the use of vast areas of land to treat runoff.
Sand filters have been used for many years for treatment of runoff, water and wastewater. The Austin Texas sand filter was one of the first used for stormwater runoff purposes. The performance of sand filters shows a high degree of variation in their pollutant removal efficiencies and it is highly susceptible to clogging. Sand filters generally have a high maintenance burden and the surface of the sand filter must be continually cleaned. Generally, sand filters do not remove nitrogen, and usually generate nitrogen in the form of nitrates though the nitrification of organic matter trapped in the sand media.
Of particular interest, in regard to the present invention, is the prior art BMP known as bioretention or sometimes commonly referred to as a xe2x80x9crain gardenxe2x80x9d. This practice was first analyzed and described in the xe2x80x9cBioretention Feasibility Analysisxe2x80x9d, Prince George""s County Government, May 1992. A bioretention design manual described the recommended criteria for the construction and maintenance of the BMP entitled the xe2x80x9cDesign Manual for the Use of Bioretention in Stormwater Managementxe2x80x9d, Prince George""s County Government, June 1993. Bioretention is described as an experimental method to treat stormwater runoff by filtering runoff through the soil and facultative plants (plants that can tolerate wet and dry conditions) to remove pollutants. The 1993 design manual provides some guidance on many aspects of the concept such as its use and purpose, locating bioretention facilities, minimum sizing guidelines, preferred plant materials, plant maintenance guidelines, soil guidelines, mulch criteria, ecological considerations, infiltration/flow rates, flow control, pollutant removal mechanisms and other design guidelines. Prince George""s County Government developed the bioretention practice to allow for greater use and treatment of stormwater runoff within the green space or landscaped areas of residential, industrial and commercial properties. Bioretention maximizes the use of green space for storage and treatment of stormwater runoff. Runoff can be diverted to a bioretention BMP located in the landscape where runoff is ponded at shallow depths (6 inches or less) passing through the mulch, soil and plant complex thus removing pollutants and allowing the treated runoff to infiltrate into the ground. This design is essentially an enhanced infiltration BMP where the filtered water is allowed to infiltrate into the ground. The bioretention system is designed to occupy about 5% to 7% of a site area to control the first xc2xd inch of runoff.
The 1993 bioretention design guidelines describe the BMP as having a shallow ponding area 6 inches deep or less, a variety of facultative plants both woody and herbaceous, a mulch layer of 2 to 3 inches, a 4 foot deep layer top soil and 1 foot of sand. The facility is excavated and filled with the materials previously mentioned. The facility is de-watered by water percolating into the surrounding ground or through evapotranspiration. The use of bioretention as described in the 1993 guideline is limited to soils with high infiltration rates and good drainage.
For the bioretention system to function properly, aerobic conditions must be maintained. Bioretention systems require good drainage and the free flow of oxygen into the soil for the health of soil microbes and plant material. The plants in the system are upland plants as opposed to wetland plants. If soils are allowed to stay wet or soggy for very long periods, anaerobic (without oxygen) conditions will develop. Under these conditions the plants and microbes will be deprived of oxygen which will limit growth, functions or cause the plants to die. As long as aerobic (with oxygen) conditions persist, the soil and microbe complex react with pollutants and nutrients making them available for plant uptake. Bioretention BMP""s were designed to make use of upland plants to remove pollutants where the soil mulch and plants act together as a system to hold, transform and metabolize the pollutants.
Pollutants are removed from stormwater runoff in the bioretention BMP by many physical, chemical and biological processes as the contaminated runoff moves through the mulch, soil, microbes and plant filter system. Suspended soils are removed throughout the process of sedimentation as runoff is allowed to pond at shallow levels above the filter media. Suspended soils are removed by filtration as the runoff passes through the soil complex. Removal of organic compounds that cause a biological oxygen demand (BOD) is accomplished by microbial degradation, filtration and sedimentation, nitrogen is removed through nitrification and plant uptake, phosphorous is removed through adsorption, sedimentation and precipitation. Heavy metals are removed through sedimentation, precipitation, adsorption and plant uptake.
Since the introduction of the bioretention in 1993, the success of the BMP has been mixed. Prince George""s County released the 1993 design guideline and described the BMP as only experimental, encouraging others to improve upon the design. The limited success of the original bioretention design is in part due to the lack of specific design standards, construction guidelines and maintenance details provided in the 1993 guidelines. This lack of specificity in the 1993 design manual was due to the fact that bioretention was a new and experimental practice. The County did not know precisely how to optimize the hydraulic and pollutant removal functions of the bioretention BMP. The lack of specificity in the 1993 design manual required inexperienced designers to rely on their limited knowledge and expertise concerning the BMP to maximize the effectiveness of the design to ensure success. Variations in the soil mix, infiltration rates, plant materials and design applications allowed for uncertain and varying results in the performance of the bioretention BMP.
Recognizing the limitations of the 1993 design and in an attempt to improve the reliability of the bioretention BMP in June of 1998, Prince George""s County completed the study xe2x80x9cOptimization of Bioretention Design for Water Quality and Hydrologic Characteristicsxe2x80x9d to investigate bioretention pollutant removal capabilities and mechanisms. In June of 1998, Prince George""s County, based on the study findings and several years of experience, issued a general design guideline update for bioretention. These 1998 design guidelines recommended a number of modifications to the 1993 design guidelines. Although these recommendations represent an improvement in the design, they still lack specificity in the application and design and rely on the designer""s own limited knowledge and experience, or lack of it, to design this still experimental practice. These current design guidelines still lack specificity in many design aspects allowing for a high degree of variation in the performance of the bioretention BMP.
A severe limitation was placed on the bioretention system in the 1993 design guideline requiring reliance on the infiltration capabilities of the in-situ soils in which the facility was constructed and used to de-water the system. To ensure some degree of success the device could only be used where infiltration rates where higher than 0.5 inches per hour. Furthermore the design allowed great variation in the amount of clay allowable in the soil media, of up to 25%. Experience showed that this high of rate of clay content slowed the infiltration of water through the systems to such a rate as to create anaerobic conditions killing the plants. When the designer chose a soil mix with high clay content and in-situ soils with a low permeability or infiltration, anaerobic conditions resulted killing the plants. The 1998 guideline update reduces the clay content to a maximum of 10% which is also very high. Since this is only a guideline, there is nothing to prevent the designer from using material with higher clay content. High clay content even at 10% will cause the soil to retain too much water and affect the performance of the plants and microbes in the facility.
Another limitation of the current design guideline is the slow and prolonged filtration rates of the water passing through the bioretention BMP. The recommended minimum infiltration rate of the soil is 0.5 inches/hr. with the facility draining within 3-4 days. This long drying out period severely limits the types of plants. It also means the system would not dry out and be ready to receive water from the next storm if the event occurred at an interval more frequent than 4 days. Furthermore, the long retention times require designing very large facilities to treat larger runoff volumes for a more frequent storm interval. The design guidelines combined with poor designs, inconsistency in the soil used, poor in-situ soil infiltration rates, improper application and use of the facility, and excessive ponding times have resulted in continued significant failures of the BMP.
Furthermore the 1998 bioretention study results showed that the bioretention design resulted in the inability of the system to remove nitrates and actually increased the amount of nitrates in the filtered water. This is in part due to the breakdown of organic nitrogen to nitrates (nitrification). Under the aerobic conditions of the bioretention filter the nitrates cannot be converted to nitrogen gas (denitrification). The study showed that nitrate levels could actually increase above the levels of the incoming contaminated water. High levels of nitrates in ground water can be a serious public health threat. Nitrates cause xe2x80x9cblue babies syndromexe2x80x9d or methemoglobinemia, which prevents oxygen from getting to the blood.
One design of a bioretention facility is to excavate a hole into the existing soils and back fill with the prepared soil mix. In cases where the facility is located adjacent to the roadways, sidewalks and buildings the disturbance of the soil around these structures, settling of the soils within the bioretention area and the flow or seepage of water into the ground around these structures can affect the structural integrity of adjacent structures. Sinkholes could develop as soil particles are carried away by ground water seepage and piping. Slope failure at the edges of the bioretention area could occur as the soils settle and lose the ability to support adjacent soils. This design contains no structural container and allows for piping or erosion of soils from around adjacent structures, buildings, roadways and sidewalks destabilizing their integrity.
The above-described bioretention design purpose is to treat the first flush (sometime defined as the first xc2xd to 1 inch of runoff). In some studies, the first flush of runoff from a site has been shown to contain higher concentrations of pollutants. This is true for some pollutants and in some situations but not all pollutants nor is it true for all situations or areas of the United States. For example, oil grease and sediment may flow off the surface at concentrations that are dependent on the duration, intensity and velocity of the storm event and have no relationship to the first flush volume. Systems designed for first flush treatment may not be capable of treating flows for long duration events or pollutants that continuously flow from the land over the entire storm event.
Stewart U.S. Pat. No. 5,322,629 discloses a chamber for treating stormwater runoff. The filter media in Stewart comprises a mature deciduous leaf compost and this filter media is drained using a drain field system. Stewart composts the filter media so as to prevent release of nitrogen and phosphorous. In the present invention, which is capable of treating domestic wastewater, industrial wastewater, as well as stormwater runoff, the source of organic matter within the soil mixture may include a wide range of non-composted materials such as wood mulch, yard wastes, shredded paper or cardboard. In addition, the present invention uses live plants growing within the filter media to treat the water and remove water from the media. There are other differences which will be apparent.
It is an objective of this invention to standardize the performance and improve upon the design of the basic bioretention BMP, to take it out of the realm of experimental devices and develop a reliable, dependable, more effective, low maintenance and structurally sound device easy to construct and maintain. The embodiments of this invention standardize the soil media, place the facility in a concrete container, increase the ability to remove nitrogen with the addition of an anaerobic denitrification chamber and system capable of treating large volumes of runoff over a greater period of time. The objective of this invention is to improve and advance the art of bioretention by improving its reliability, pollutant removal capability, eliminate the problems of danger to the structural integrity of surrounding structures, increase its capacity to treat greater volumes of runoff and to increase the type, variety, aesthetics and habitat value of plants which can be used in the facility. This invention can be used for a variety of contaminated waters including urban stormwater runoff, agricultural runoff and domestic agricultural, industrial and commercial wastewater.
Of particular interest to this invention is the use of filter devices to treat and remove pollutants from stormwater runoff. Filters use organic materials, inorganic materials and living organisms individually or in variety of combinations to provide a media for a wide range of physical, chemical and biological mechanisms to remove, capture and transform pollutants.
The invention preferably embodies a two-stage biologically active filtering and treatment system to remove pollutants from contaminated water prior to being discharged to the surface, drainage systems or into/onto the ground. The first stage is a mulch, soil, microbe and plant complex filter that removes pollutants using a variety of physical, chemical and biological mechanisms under predominately aerobic conditions. It has been demonstrated that when contaminated water such as stormwater runoff or wastewater is brought into contact with or filtered/percolated through a soil/plant complex, that pollutants and nutrients are removed. One aspect of this invention is to provide for a uniform and standardized soil mixture to optimize the flow rate and pollutant removal. The mulch, soil and plant filter complex is contained in an engineered structure of specific dimensions and geometry to provide for adequate flow and pollutant removal. Water enters the first chamber percolating through the filter media by gravity and the plant roots and soil media remove pollutants. The water is then collected in a horizontal collector under drain pipe at the bottom of the chamber. The treated water then flows through the horizontal pipe to a vertical pipe where it is once again filtered by shredded mulch contained in the vertical pipe and suspended in the vertical pipe by a wire screen retaining device. This mulch material filters the water and allows it to pass to the second water-filled chamber where pollutants are further removed by physical and microbiological processes under anaerobic conditions. Another aspect of this invention is that the second anaerobic chamber treatment chamber is designed with a specific dimension and geometry to achieve the desired level of removal of pollutants. Under anaerobic conditions nitrate is converted to nitrogen gas, a process known as denitrification. The second chamber or vault is designed with a series of baffles to maximize the water flow and retention time for treatment through a second chamber. After the water passes through the second chamber, it is then discharged to the surface, drainage systems or into/onto the ground.
As an alternative embodiment, the first chamber may be utilized without the second chamber in a manner to be described.